A light-emitting element using a light-emitting material features thinness, lightness in weight, and the like, and is expected to be applied to a next-generation flat panel display. Further, because of its self-luminous properties, such a light-emitting element using a light-emitting material seems to be advantageous over conventional liquid crystal displays (LCDs) in wide viewing angle and high visibility.
A light-emitting element seems to emit light by the following manner; a voltage is applied to a pair of electrodes which interpose a light-emitting layer therebetween so that holes injected from an anode and electrons injected from a cathode are recombined at luminescence centers of the light-emitting layer to excite molecules, and the excited molecules discharge energy in returning to a ground state, thereby light is emitted. Note that excited states that are generated by recombination are a singlet excited state and a triplet excited state. Light emission is considered to be possible through either state of the singlet excited state or the triplet excited state. The light emission occurring when excited molecules return from the singlet excited state to the ground state directly is called fluorescence, and the light emission occurring when excited molecules return from the triplet excited state to the ground state is called phosphorescence.
Such a light-emitting element has many problems depending on materials in order to enhance its characteristic. In order to overcome the problems, reformation of the element structure, development of the material, and the like have been conducted.
For example, an anthracene derivative emitting green light is disclosed in Patent Document 1 (: United States Published Patent Application No. 2005/0260442).